Just had a Santiago replaced for whiteworm, randomly appeared in the middle of the spoke. They couldn't find any damage to pin it on so replaced the wheel. Not impressed since i run winter tyres and worked out it lasted just 14K miles before whiteworm appeared. They'll be getting powdercoated if required in the future, not having this sh!t every couple of summers.
Always keep the wheels clean too, even if i haven't got time for the whole car.